How can i add a texture to billboard particles?
As answered in the bug report by Brecht, billboards is a feature that is not supported by cycles and isn't planned to be added any time soon.

To get billboards working you need to be using the blender internal render engine. By default the material used for particles is the same material used on the object emitting the particles, you can change the material used for the billboards in the particle settings in the render panel.